The present invention relates to short circuit protection systems for alternating current (AC) power supplies and more particularly relates to short circuit protection systems for AC power supplies used with electrical controls for heating and/or cooling equipment such as heat pumps.
Normally, an electrical control for heating and/or cooling equipment such as a heat pump, includes a power supply, usually in the form of a transformer, which is selectively connected to output circuits for controlling various functions of the equipment. Usually, the power supply includes a fuse for preventing undesirable, large current flows in the control circuitry if there is a short circuit condition in one of the output circuits. For safety reasons, the fuse is inaccessible and, if the fuse is blown, it is necessary to replace the entire power supply to restore operation of the equipment. This situation may be avoided by using a short circuit protection circuit which protects the power supply and fuse against a short circuit condition in the load.
Short circuit protection circuits for protecting direct current (DC) power supplies from a short circuit condition in a load are well known. These circuits include a resistance device electrically connected in series between the power supply and the load. Current flow through the device is monitored to detect a short circuit condition in the load. An unusually high current flow through the device indicates the existence of a short circuit condition in the load.
A short circuit protection circuit may be included with or added to an electrical control for heating and/or cooling equipment such as a heat pump. If the control utilizes an AC power supply then current sensing protection circuits, similar to the circuits for protecting a DC power supply, may be used to protect the AC power supply from a short circuit condition in a load connected to the AC power supply. However, use of a short circuit protection circuit increases the cost of the control and may reduce the output voltage of the protected AC power supply because of the placement of a resistance device between the power supply and the load. Furthermore, random fluctuations in power supply voltage or frequency, and/or variations in load current may cause undesirable triggering of the protection circuit.